1. Field of the Invention
This invention pertains generally to semiconductor nanowires, and more particularly to diluted magnetic semiconductor nanowires.
2. Incorporation by Reference of Publications
The following publications referenced herein using numbers inside brackets (e.g., [1]) are incorporated by reference herein in their entirety:    [1]: S. D. Sarma, Ferromagnetic semiconductor: A giant appears in spintronics, Nature Mater. 2, 292-294 (2003).    [2] S. P. Pearton et al., Wide band gap ferromagnetic semiconductors and oxides, J. Appl. Phys. 93, 1-13 (2003).    [3] H. Akinaga, H. Ohno, Semiconductor spintronics, IEEE Trans. Nanotech. 1, 19-31 (2002).    [4] T. Dietl, Ferromagnetic semiconductors, Semicond. Sci. Technol. 17, 377-392 (2002).    [5] Malajovich, J. J. Berry, N. Samarth, D. D. Awschalom, Persistent sourcing of coherent spins for multifunctional semiconductor spintronics, Nature, 411, 770-772 (2001).    [6] H. Ohno, Making nonmagnetic semiconductor magnetic, Science, 281, 951-956 (1998)    [7] S. Wolf et al., Spintronics: a spin based electronics vision for the future, Science, 294, 1488-1495 (2001).    [8] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Zener model description of ferromagnetism in zinc-blende magnetic semiconductors, Science, 287, 1019-1022 (2000).    [9] K. H. Kim et al., Magnetotransport of p-type GaMnN assisted by highly conductive precipitates, Appl. Phys. Lett. 82, 1775-1777 (2003).    [10] H. Hori et al., High T. ferromagnetism in diluted magnetic semiconducting GaN:Mn films, Physica B 324, 142-150 (2002).    [11] M. C. Park et al., Room temperature ferromagnetic (Ga,Mn)N epitaxial films with low Mn concentration grown by plasma-enhanced molecular beam epitaxy, Solid State Commun. 124, 11-14 (2002).    [12] M. L. Reed et al., Room temperature ferromagnetic properties of (Ga, Mn)N, Appl Phys. Lett. 79, 3473-3475 (2001).    [13] M. Zajac et al., Paramagnetism and antiferromagnetic d-d coupling in GaMnN magnetic semiconductor, Appl. Phys. Lett. 79, 2432-2434 (2001).    [14] Y. Soon et al., Optical and magnetic measurements of p-type GaN epilayers implanted with Mn+ ions, Appl. Phys. Lett. 81, 1845-1847 (2002).    [15] N. Theodoropoulou et al., Magnetic and structural properties of Mn-implanted GaN, Appl. Phys. Lett. 78, 3475-3477 (2001).    [16] M. E. Overberg et al., Indication of ferromagnetism in molecular-beam-epitaxy derived n-type GaMnN, Appl. Phys. Lett. 79, 1312-1314 (2001).    [17] Y. Xia, P. Yang Eds., Special issue on one-dimensional nanostructures, Adv. Mater. 15 (5), 2003.    [18] H. Choi, J. Johnson, R. He, S. Lee, F. Kim, P. Pauzauskie, J. Goldberger, R. Saykally, P. Yang, Self-organized GaN quantum wire lasers, J. Phys. Chem. B, 2003, In press.    [19] Z. L. Wang, J. S. Yin, Y. D. Jiang, EELS analysis of cation valence states and oxygen vacancies in magnetic oxides, Micron, 31, 571-580 (2000).    [20] Shon, Y et al., Optical characterization of Mn+ ion implanted GaN epilayers, J. Crys. Growth, 245, 193-197 (2002)    [21] K. H. Kim et al. Enhanced carrier-mediated ferromagnetism in GaMnN by codoping Mg, Appl. Phys. Lett. 82, 4755-4757 (2003).    [22] L. J. Lauhoni, M. S. Gudiksen, D. Wang, C. M. Lieber, Epitaxial core-shell and core-multishell nanowire heterostructures, Nature, 420, 57-161 (2002).    [23] R. He, M. Law, R. Fan, F. Kim, P. Yang, Functional bimorph composite nanotapes, Nanolett., 2, 1109-1112 (2002).    [24] Z. Zhang et al. Magnetotransport investigations of ultrafine single-crystalline bismuth nanowire arrays, Appl. Phys. Lett. 73, 1589-1591 (1998).    [25] Jin, S. et al., Thousand-fold change in the resistivity in magnetoresistive La—Ca—MnO films, Science, 264, 413-415 (1994).    [26] Shimakawa, Y, Kubo, Y & Manako, T., Giant magnetoresistance in Tl2Mn2O7 with the pyrochlore structure, Nature, 379, 53-55 (1996).    [27] Solin, S. A.; Thio, T.; Hines, D. R.; Heremans, J. J., Enhanced room-temperature geometric magnetoresistance in inhomogeneous narrow-gap semiconductors, Science, 289, 1530-1-532 (2000).    [28] Y. Matsumoto et al., Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science, 291, 854-856 (2001).    [29] T. Fukumura et al., An oxide-diluted magnetic semiconductor: Mn-doped ZnO, Appl. Phys. Lett. 75, 3366-3368 (1999).    [30] Manala N. Sidis Y, DiTusa J. F., Aeppil G Young, D. P., Fisk, Z. Magnetoresistance from quantum interference effects in ferromagnets, Nature, 404, 581584 (2000).
3. Description of Related Art
Current information technology relies on two independent processes: charge-based information processing (microprocessors) and spin-based data storage (magnetic hard drives). [1-5] The perspective of simultaneously manipulating both charge and spin in a single semiconductor medium leads to the exciting area of spintronics. [1-7] Among many others, diluted magnetic semiconductors (DMSs) represent the most promising candidates for such applications. Theoretical studies indicate that transition metal doped GaN possesses ferromagnetic transition temperature higher than room temperature, which would be advantageous for many of the proposed spintronic applications. [8] Many experiments have already been carried out to demonstrate such a hypothesis, [9-11] although significant controversy exists over the possible magnetic impurity phase separation for these thin-films. [12-16]. Moreover, intrinsic defects in these films originated from the non-equilibrium molecular beam epitaxial growth process hinder a fundamental understanding of the ferromagnetism in these materials.
On the other hand, the miniaturization of electronic devices represents an everlasting trend for both industrial manufacture and academic research. Among many other materials, nanotubes and nanowires are being actively explored as possible building blocks for electronic devices of sub 100 nm and smaller. [17] The controlled fabrication and fundamental understanding of low-dimensional ferromagnetic semiconductor nanostructures is thus crucial to the development of semiconductor-based spintronic devices and spin-based quantum computation schemes.
Although progress has been made in the understanding of DMS quantum wells and dots, [1-7] studies on DMS quantum wires are still at a nascent stage. Dimensionality and size are known to play significant roles in determining various properties of the system. In this regard, one dimensional DMS systems in nanometer scale, i.e., DMS nanowires, are expected to have interesting magnetoelectronic properties and could be good candidates for realizing spintronic devices for several reasons. First, nanowires themselves are attractive building blocks for nanoscale electronic and optoelectronic devices; second, magnetic nanowires could act as spin filters to supply spin polarized carrier currents and can have large magnetic anisotropy energy; third, carriers could be confined in the radial direction of nanowires and, therefore, high carrier concentrations and efficient injection of spin polarized carriers could potentially be achieved.
Synthesis of DMS nanowires, however, represents a challenging issue, which has only recently been achieved in an epitaxial nanotape geometry. To carry out meaningful investigation on DMS nanowires, the ideal wires should be single crystalline and the transition metal dopant must be homogeneously distributed without phase separation. The synthetic challenge resides in the limited transition metal equilibrium solubility in semiconductors as well as intrinsic difficulty in nanocrystal doping. Processes like molecular beam epitaxy (MBE) and implantation have shown limited success in preparing DMSs from III-V semiconductors, particularly the GaN system. [9-16]